Underground excavations are usually supported by, amongst other things, bolts or anchors inserted into bore holes drilled into the roof and sides of the excavations. The tensioned bolts cause the surrounding rock to be compressed thereby assisting in the support integrity of the opening. Oftentimes, in addition, cementitious materials (grout, shotcrete, etc.) or resin-type materials are inserted into the void between the bore hole and bolt to impart greater structural anchorage as well as to protect the bolt from the corrosive environment of the rock face.
It appears to be universally accepted that a one-pass ground support system is an ideal technique when rock bolts are employed. However, it is difficult and expensive to provide a grouted bolt which also provides immediate support and simultaneously allows for a fast turn-around time. Present techniques include an initial bolt installation followed by a second or even third reconditioning program. The additional steps have a detrimental effect on the advance rate of the heading.
To reduce interference with the advance rate, the usual approach is to install a mechanical bolt plus screen system and then follow up later with a secondary system such as grouted rebar, grouted mechanical bolt, Swellex.TM., Split-set.TM., etc. Each of these has one or more disadvantages such as expense, insufficient holding power, early corrosion, uncertain quality control, etc.
Mechanical bolts are considered to be a stiff system with a point anchorage at each end. Anchor shell breakage or slippage at one end or ground break-up or bolt failure at the other end makes the bolt useless. These bolts are also prone to corrosion. However, they are frequently used because they are quick to install, provide immediate support, are tensioned and can pull up protective screening close to the roof.
On the other hand, grouted or resin surrounded bolts are corrosion resistant and the full length grouting of the bolt provides an important unifying competence to the rock mass.
The grouted mechanical bolt is a common example of current bolt designs but it is very expensive because of initial hardware cost as well as intricate installation procedures. See, for example, U.S. Pat. No. 3,316,797 (Williams). Another system employs a threaded rebar which is installed with a combination of resin cartridges to provide a quick-set anchor followed by machine tensioning followed by the setting of the slower speed cartridges. This is an expensive system but relatively fast. However, quality control can be difficult to maintain.
A basic problem with grouted bolts lies with the inconvenience and damage to a grout tube or tubes which must necessarily protrude from the drill hole and through the rock bolt plate(s).
From the foregoing it is apparent that an ideal bolt should:
(1) provide immediate support; PA1 (2) provide full column loading as opposed to point loading; PA1 (3) be corrosion resistant; PA1 (4) be quickly installed; PA1 (5) use cheaper cement grouts versus resins; PA1 (6) be easily grouted; and PA1 (7) be reasonably priced.
There are of course, certain situations where an initial, modest cost mechanical bolt installation will last for many years and a reconditioning program not required. However, when experience dictates that a reconditioning program will probably be required, it also implies that ground conditions will deteriorate. This deterioration adds to the overall safety hazard and points to the obvious--that a good initial program is imperative.